This invention relates to a technology for reducing artifacts resulting from movement of an inspection object during imaging in a magnetic resonance (MR) imaging system. More particularly, it relates to a method of executing an imaging operation in such a manner as to track movement of an inspection object which can be regarded as a rigid body.
U.S. Pat. No. 5,042,485 describes a method which executes an imaging operation in such a manner as to track vertical motion of the heart by changing a frequency of a radio frequency (RF) magnetic field. A slice section which is desired to be imaged moves up and down in accordance with pulsation. Therefore, this method so sets the frequency of the RF magnetic field as to track fluctuation of the resonance frequency of the imaging desired section and executes imaging so as to track the vertical motion of the heart.
Mark Hedly et al., IEEE Transactions on Medical Imaging, Vol. 10, No. 4 (1911), pp. 548-553 describes a technology which removes artifacts occurring on an image by phase correction of measurement signals when an inspection object which can be regarded as a rigid body, such as the head, causes parallel motion with respect to the imaging section. Hereinafter, explanation will be given by using symbols which are somewhat different from those used in the reference. Signal data S (K, L) when the rigid body causes parallel motion with the imaging section can be expressed as follows by signal data S.sub.o (K, L) when motion does not exist: EQU S(K, L)=S.sub.o (K, L).multidot.exp (2.pi.i.multidot.(K.multidot..DELTA.X.sub.1 +L.multidot..DELTA.Y.sub.1)) (1)
where K is frequency spatial coordinate value in read direction,
L is frequency spatial coordinate value in encode direction,
.DELTA.X.sub.1 is a moving distance in read direction,
.DELTA.Y.sub.1 is a moving distance in encode direction.
Therefore, when phase-corrected signal data S' (K, L) is generated as expressed by equation (2), equation (3) can be obtained: EQU S'(K, L)=S(K, L).multidot.exp(-2.pi.i.multidot.(K.multidot..DELTA.X.sub.1 +L.multidot..DELTA.Y.sub.1)) (2) EQU S'(K, L)=S.sub.o (K, L) (3)
Accordingly, the phase-corrected signal data S' (K, L) becomes equal to the signal data S.sub.o (K, L) when no motion exists.
The Mark Hedley et al. reference has a characterizing feature in that detection of the moving distances .DELTA.X.sub.1, .DELTA.Y.sub.1 is estimated from the artifacts appearing on the resulting reproduced image. In other words, an image is formed by removing the artifacts appearing on portions at which the inspection object does not originally exist, and the moving distances .DELTA.X.sub.1 and .DELTA.Y.sub.1 are estimated on the basis of the signal data generated from the image from which the artifacts are removed and on the basis of a phase difference of the original signal data. Further, the artifacts appearing on the portions at which the object does not originally exist are removed for the image reproduced from the signal of equation (2) for which correction is made, and correct estimation of .DELTA.X.sub.1 and .DELTA.Y.sub.1 is sequentially carried out.
H. W. Korin et al., Society of Magnetic Resonance in Medicine, 9th Annual Metting and Exhibition, (1990) p. 560 provides correction means which is effective when an inspection object which can be regarded as a rigid body, such as the head, rotates inside an imaging slice plane. This reference pays a specific attention to the fact that when the inspection object rotates inside the slice plane, the measurement signal data, too, undergoes similar rotation on a K space. Therefore, the reference method monitors the rotation by external markers, and rotates and interpolates the measurement signals containing the rotation on the K space so as to thereby obtain a corrected image free from influences of rotation.
J. P. Felmlee et al., Society of Magnetic Resonance in Medicine, 10th Annual Meeting and Exhibition, Works in progress, (1991), p. 1227 describes a technology which detects a position moving distance in a read direction inside an imaging slice plane from zero encode data referred to as "navigator echo", and applies correction to the image echo. This method sets the read direction to a transverse direction in which movement of the object can be anticipated in advance, detects the position moving distance by the navigation echo, corrects the image echo, and obtains an excellent spine image.